1. Field of the Invention
The present invention relates to a speaker and more particularly to a speaker in which a piezoelectric body made of ceramic is used and adapted for use as a center channel woofer in a three dimensional ("3D") sound system such as a stereo system.
2. Description of Related Art
A speaker having a sounding body in the form of a piezoelectric member made of ceramic causes an amplitude of sound waves generated thereby to be insufficiently small. Accordingly, a speaker having the piezoelectric sounding body has not been used as a woofer for generating low frequency sound.
The inventor of the present invention has invented a speaker for functioning as a woofer and which comprises a piezoelectric body for generating low frequency sounds. This speaker is the subject of Japanese Patent Application No. Hei. 8-163854 and U.S. patent application Ser. No. 08/867,288. The disclosure of U.S. patent application Ser. No. 08/867,288 is hereby incorporated by reference.
The speaker disclosed in Japanese Patent Application No. Hei. 8-163854 and U.S. patent application Ser. No. 08/867,288 will be explained with reference to FIG. 3 herein. The speaker 1 includes a main body 2 having open sections at both ends thereof. Ringed dampers 6 are provided, respectively, at the open sections at both ends of the main body 2 and sounding bodies 3 are attached to the dampers 6. The sounding bodies 3 are secured so that the inside of the main body 2 is kept air-tight. A first opening 2a is provided at substantially the middle part of the side of the main body 2.
Further, lid members 4 are provided on the outside of the two sounding bodies 3 so as to cover the open sections at the both ends of the main body 2 and are secured so that the inside of the lid member 4 is kept air-tight. A second opening 4a is provided around substantially the middle of the bottom of the lid members 4. Sound absorbers 5 for absorbing and damping components of sound waves in the high sound range are preferably provided within the main body 2 and within the lid members 4.
The sounding bodies 3 preferably comprise a disc-like vibrating plate 3a made of metal and disc-like piezoelectric elements 3b are secured as a vibration source at the center part of both main surfaces of the vibrating plate 3a on a concentric circle of the vibrating plate 3a so as to have a bimorph structure. Although not shown specifically, the piezoelectric element 3b is constructed preferably by forming electrode films on both main surfaces of the disc-like piezoelectric body.
Both of the first opening 2a and the second opening 4a are smaller than an area of the vibrating plate 3a of the sounding body 3.
A first one of the two sounding bodies 3 is connected to a first input terminal (not shown) provided on the outside of the main body 2 preferably via a lead wire (not shown) end a second one of the two sounding bodies 3 is connected to a second input terminal (not shown) provided on the outside of the main body 2 preferably via another lead wire (not shown).
When an electrical signal is input from an amplifier (not shown) to the first input terminal, a corresponding one of the two sounding bodies 3 vibrates and when an electrical signal from the amplifier (not shown) is input to the second input terminal, the other of the two sounding bodies 3 vibrates.
The sounding bodies 3 are driven so that pressure within the main body 2 is increased/decreased at the same time by the two sounding bodies 3. That is, when one channel signal, e.g. a right channel signal, of a stereo signal is input to one input terminal, the other channel signal, i.e. a left channel signal, of the stereo signal is input to the other input terminal.
By inputting signals as described above, sound waves are generated within the main body 2 from the two sounding bodies 3 and are emitted from the first opening 2a of the main body 2. Sound waves are generated also within enclosed spaces located between the main body and each of the lid members 4 from the two sounding bodies 3 and are emitted from the second opening 4a of the lid member 4.
Because the area of the first opening 2a of the main body 2 is smaller than the area of the vibrating plate 3a of the sounding body 3 and the amplitude of the sound waves emitted from the first opening 2a of the main body 2 is larger than amplitude of the sound waves generated within the main body 2 from the two sounding bodies 3, the low frequency sound output therefrom is significantly increased. Further, because the sound waves in the middle/high frequency sound range generated from the two sounding bodies 3 have phases which are opposite from each other and are canceled within the main body 2, only the low frequency sound range is maximized and output from the first opening 2a.
Consequently, the speaker 1 generates a full low frequency sound range by using the piezoelectric body and without using a large vibrating plate, a large case or a large baffle plate.
Furthermore, because the sound absorbers 5 absorb and damp the high frequency sound range component of the sound waves generated within the main body 2 and within the sealed inner portions between the lid member 4 and the main body from the two sounding bodies 3, the low frequency sound range of the output sound waves is maximized as a result.
However, because each of the sounding bodies is comprised of the plate-like piezoelectric body, a vibration area of each sounding body is divided into a piston vibration area and a split vibration area depending on a frequency of signal input to the sounding body. That is, the speaker functions in the piston vibration area when the driving frequency is equal to a frequency of a standing wave and the speaker functions in the split vibration area when the driving frequency is above the frequency of the standing wave.
In a speaker utilizing such a resonance phenomenon, although the resonance phenomenon occurs on a sound wave generated by the piston vibration, no resonance phenomenon occurs on a sound wave generated by the split vibration because such a sound wave inherently contains much distortion.
When no resonance phenomenon occurs, the low frequency sound range is not stressed or featured. Instead, the middle/high frequency sound range is stressed or featured and the speaker cannot function as a woofer.
In contrast, the speaker of FIG. 3 including the piezoelectric sounding body functions as a woofer by utilizing a resonance phenomenon defined by the volume of the main body and the shape of the first opening and a resonance phenomenon defined by the volume of the lid members and the shape of the second openings.
When sound waves caused by the split vibration generated from the two sounding bodies exist within the main body of the speaker of FIG. 3, the sound waves in the middle/high frequency sound range are canceled within the main body and no sound waves in the middle/high frequency sound range caused by the split vibration are emitted from the first opening because the phases of the sound waves in the middle/high frequency sound range generated respectively from the two sounding bodies have phases which are opposite to each other.
However, the sound waves generated inside the lid members is different from that generated inside of the main body. Only the sound waves caused by the split vibration generated from one of the two sounding bodies exists within the lid members and the sound waves in the middle/high frequency sound range caused by the split vibration are not canceled.
Accordingly, the sound waves in the middle/high frequency sound range caused by the split vibration are emitted from the second opening of the lid members and are not canceled. This high frequency sound range cannot be absorbed fully by the sound absorber, so that sound waves both in the high frequency sound range and the low frequency sound range are emitted from the second openings. Further, because the sound waves caused by the split vibration do not create a resonance phenomenon, the sound of the speaker for the woofer is emitted with much distortion and very low sound level or amplitude.